Some master cylinder devices, such as a master cylinder device disclosed in the following patent literature, can pressurize a brake fluid depending on a pressure of a brake fluid introduced from a high pressure source irrespective of an operation force for operating a brake operation member by a driver. This master cylinder device includes a pressure receiving piston which moves forward by a pressure of a brake fluid in an input chamber, that is, a fluid chamber into which the brake fluid is introduced and which then pressurizes a brake fluid, and an input piston which is fitted in the pressure receiving piston at a blind hole thereof being open rearward and which moves forward by a brake operation. Between a bottom portion of the blind hole and a front end face of the input piston, there is generally formed a fluid chamber filled with a brake fluid (hereinafter, referred to as an “inter-piston chamber”, where appropriate). Therefore, the pressure receiving piston and the input piston can move independently from each other.    Patent Literature 1: JP-A-2010-929 (P. 20, FIG. 3)